In the image forming apparatus of electrophotographic printing type, the surface of the photoconductor which is the image supporting medium is subjected to the charging processing by applying the charge of a predetermined polarity by the discharging, and the charged photoconductor surface is exposed to light so that an electrostatic latent image is formed. The toner which is charged with the same polarity as in the charging processing is supplied to this electrostatic latent image, and the toner image is formed. After that, the toner image formed on the photoconductor is transferred to the recording paper etc., and it is fixed to the record paper by applying the heat and the pressure.
Since the toner which is not used for the transferring remains on the photoconductor surface after the toner image is transferred, the photoconductor surface is cleaned by the cleaning components, such as the cleaning blade and the cleaning brush, before going into the following charging process.
In recent years, an improved method of charging the photoconductor surface in the above-mentioned image forming apparatus has been put in practical use. In the improved method, the charging roller which is formed in the shape of a roller by the conductive component is made to contact or approach the photoconductor surface, and the charging voltage between the charging roller and the photoconductor is supplied in this state, so that the charging device which charges the surface of the photoconductor can attain a low ozone concentration and low power consumption.
However, the remaining toner which remains on the photoconductor surface after the toner image is transferred is not completely removed in the cleaning process, and such toner may reach the region in the vicinity of or in contact with the charging roller. There is the problem that the remaining toner after the toner image transfer adheres to the charging roller.
Suppose that the toner which is contained in the remaining toner and charged to the same charging polarity is referred to as the regular polarity toner. When stirring the toner with the developer, the so-called reverse charging toner exists. The reverse charging toner is not charged to the regular polarity but charged to the reverse polarity.
The toner charged to the regular polarity repels the charging roller due to the static electricity, and it does not adhere to the charging roller surface. However, the reverse charging toner and the charging roller draw each other due to the static electricity, and it easily adheres to the charging roller surface.
Moreover, besides the reverse charging toner, the foreign matter, such as the paper chip, which is charged to the reverse polarity, will easily adhere to the charging roller due to the static electricity.
With the increasing demand for high-quality, very-fine images in recent years, the toner having spherical particles with smaller particle diameters has been used increasingly in the development process. The purpose of using this toner is to make the toner adhere to the electrostatic latent image precisely.
However, the toner having spherical particles with smaller particle diameters tends to penetrate through the cleaning blade in the cleaning process, and the penetrating toner makes the charging roller surface contamination increase further.
Moreover, in order to perform the charging processing on the surface of the photoconductor uniformly, there has been proposed the method of applying the charging bias in which the AC (alternating current) voltage is superimposed on the DC (direct current) voltage.
If the AC voltage is superimposed, the photoconductor surface will be damaged and its abrasion will increase. In order to prevent the abrasion of the surface of the photoconductor, it is desirable to protect the photoconductor surface.
Then, there has been proposed the method of applying the lubricant, such as the fatty acid metal salt, to the photoconductor surface for protecting the photoconductor surface. Applying the lubricant to the photoconductor surface allows reduction of the coefficient of friction on the surface of the photoconductor, and it works also to raise the rate of the transfer of the toner image, and the efficiency of cleaning of the remaining toner by the cleaning blade, etc.
However, the external additive particles with a certain hardness, such as silica particles, which are separated from the toner are easily caught by the thin layer of the lubricant provided in the photoconductor surface. When the photoconductor surface passes through the contact surface of the cleaning blade, the blade edge will be damaged in such a case so that the breakage and abrasion of the cleaning blade may arise.
If the abrasion of the cleaning blade progresses, the amount of the toner which passes through the cleaning blade will be increased and the charging roller surface contamination will also be increased.
As mentioned above, in order to prevent the adhesion of the toner passing through the cleaning blade to the charging roller and to avoid the difficulty in performing uniform charging of the photoconductor surface, it is necessary to clean the charging roller surface adequately.
As a cleaning component of the charging roller, the sponge material, such as foaming polyurethane and foaming polyethylene, has been proposed (see Japanese Laid-Open Patent Application No. 05-297690). Moreover, as a cleaning component of the charging roller, the brush roller has been proposed (see Japanese Laid-Open Patent Application No. 2002-221883).
Any of these cleaning components is brought in contact with the charging roller surface for friction so that the adhering matter, such as the toner, is removed from the charging roller surface.
In the case of the sponge material, the adhering matter will be accumulated in the internal pores of the sponge material. In the case of the brush roller, the adhering matter will be accumulated in the internal space between the fibers of the brush roller.
However, the amount of the adhering matter that can be accumulated by such cleaning components is limited, and it is difficult to maintain the cleaning performance of the cleaning components over an extended period of time. For example, it is required for the process cartridge including the charging roller to maintain the cleaning function of the charging roller so as to be suitable to the life of the other components. The use of the cleaning mechanism including the above-mentioned cleaning component is inadequate to meet the requirement.
On the other hand, in many cases of the conventional charging rollers, the elastic body, such as rubber or elastomer, in which the electric conduction agent is dispersed, is coated to the outer periphery of the core metal. Such elastic body easily produces the setting and the deformation with the elapsed time.
For this reason, in the charging device of the non-contact type in which the photoconductor and the charging roller are placed in proximity with no contact, the gap between the photoconductor and the charging roller varies with the elapsed time, and the problem, such as image unevenness, takes place due to the change of the charging potential.
For this reason, there has been proposed the charging roller which is constructed using the inelastic body, such as a thermoplastic resin, instead of the elastic body, such as rubber or elastomer (see Japanese Laid-Open Patent Application No. 2002-132019).
However, also in the image forming apparatus equipped with the charging roller constructed using the above-mentioned inelastic body, the problem of contamination of the charging roller surface by the toner penetrating through the cleaning blade which contacts the photoconductor arises similarly. Thus, it is desirable to provide a cleaning mechanism which is able to clean the charging roller surface adequately continuously over an extended period of time.